How is your soldering? You are unlikely to be able to increase the throw at the Tx end sufficiently to get 180 degrees from a standard servo.

It is theoretically possible to modify the servo amps to achieve 180 degree travel, but it is not a job for the faint hearted or inexperienced! There are many pitfalls.awaiting the unwary. It might be easier just to try and get some 180 degree servos - if anyone sells them any more....!

Ive used a toothed belt and two pulleys of equal teeth with one pulley on the steam valve the other I have mounted on the servo.

If I can get a half turn of the servo giving me a half turn on the steam valve using the throttle channel on my Tx with the stick going from full down to full up I will have achieved my object only mini servos will fit ie 12 x 23 x 21 mm.

I have some code for a small Microchip PIC processor that extends a servo pulse to give a 180 degree rotation for using a normal servo to drive mechanical retracts. It does also slow down the output change to give a realistic retract operation.

If the servo has wiring to the potentiometer , `pot`, and to the motor as opposed to ones which are directly soldered to the PCB then you can add resistors to each end of the pot. Try values from 1K0 to 2K7 (1000 to 2700 Ohms) which will extend the travel. For flaps I have used TGY 180D`s to great effect.

Couple of thoughts. The proportional servos with 180° rotation are used by the flying cameras people, and the robot builders. They have their own websites.

One which has the dimensions you need is on the HobbyKing site, is a analogue servo, cheap. SKU is 9399000001. If you want to up its power a bit, run it on a 2s LiFe battery, 6.6 volts. But you may need a charger for this battery. I did see they were selling a B6 charger for a small price on special offer the other day. But that's about the end of the knowledge tree for me.